The so-called tandem type color printer is known as an electrophotographic color printer formed by parallelly arranging photosensitive drums corresponding to yellow, magenta, cyan and black respectively.
The tandem type color printer includes a developing roller opposed to each photosensitive drum. An electrostatic latent image is formed on the surface of the photosensitive drum. When the electrostatic latent image is opposed to the developing roller following rotation of the photosensitive drum, the developing roller feeds a toner to the electrostatic latent image. Thus, a toner image is formed on the surface of the photosensitive drum. A sheet is transported by a belt to be successively opposed to the photosensitive drums respectively. Toner images of the corresponding colors are formed on the photosensitive drums respectively and transferred to the sheet in a superposed manner, to form a color image on the sheet. On the other hand, a black toner image is formed only on the black photosensitive drum and transferred to the sheet, to form a monochromatic image on the sheet.
When the monochromatic image is formed, no toner images are formed on the remaining photosensitive drums of yellow, magenta and cyan except black, and hence the corresponding developing rollers are desirably separated from the yellow, magenta and cyan photosensitive drums respectively, to be prevented from consumption.
Therefore, an image forming apparatus including a translation cam member linearly movable in the direction of arrangement of photosensitive drums and intermediate members displaced by linear movement of the translation cam member for separating the photosensitive drums from developing rollers by pressing developing apparatuses upward is proposed as a tandem type image forming apparatus. The translation cam member so linearly moves as to switch the image forming apparatus to an all-color separation state where the developing rollers are separated from all photosensitive drums, a black contact state where the corresponding developing roller is in contact with a black photosensitive drum while the remaining developing rollers are separated from yellow, magenta and cyan photosensitive drums, and an all-color contact state where the developing rollers are in contact with all photosensitive drums.
In the image forming apparatus according to this proposal, the photosensitive drums of the four colors are integrally retained on a drum unit, which is mountable on/detachable from a main body. The developing apparatuses of the respective colors are detachably mounted on the drum unit.
In the conventional structure according to this proposal, however, the drum unit may positionally deviate from the main body due to force applied from the intermediate members to the developing apparatuses when the former presses the latter. When the drum unit positionally deviates from the main body, an image formed on a sheet also positionally deviates following this deviation.